U.S. patent application number 10/040801 was filed with the patent office on 2002-08-15 for intervertebral spacer device utilizing a spirally slotted belleville washer and a rotational mounting.
Invention is credited to Errico, Thomas J., Ralph, James D..
Application Number | 20020111686 10/040801 |
Document ID | / |
Family ID | 32718295 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020111686 |
Kind Code |
A1 |
Ralph, James D. ; et
al. |
August 15, 2002 |
Intervertebral spacer device utilizing a spirally slotted
belleville washer and a rotational mounting
Abstract
An intervertebral spacer having opposing plates seatable against
opposing vertebral bones, separated by at least one spring
mechanism, preferably a spirally slotted belleville washer having a
wide end rigidly fixed to an upper plate and a narrow end rotatably
mounted to a lower plate. The lower plate includes an inwardly
deflecting central post extending upwardly from the inner surface
of the lower plate, the post including a head that is received
through a central opening in the narrow end of the washer so that
the washer is restricted from angulation with respect to the lower
plate, but allowed to rotate with respect to the lower plate so
that the plates can rotate relative to one another. The plates can
angulate relative to one another because the washer deflects under
lateral deflection forces and return to its undeflected shape when
the forces are relieved.
Inventors: |
Ralph, James D.; (Seaside
Park, NJ) ; Errico, Thomas J.; (Summit, NJ) |
Correspondence
Address: |
Joseph P. Errico
150 Douglas Road
Far Hills
NJ
07931
US
|
Family ID: |
32718295 |
Appl. No.: |
10/040801 |
Filed: |
January 7, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10040801 |
Jan 7, 2002 |
|
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09789936 |
Feb 15, 2001 |
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Current U.S.
Class: |
623/17.13 |
Current CPC
Class: |
A61F 2002/30774
20130101; A61F 2002/30649 20130101; A61F 2220/0033 20130101; A61F
2002/30563 20130101; A61F 2002/30565 20130101; A61F 2002/30571
20130101; A61F 2002/30604 20130101; A61F 2220/0058 20130101; A61F
2002/30538 20130101; A61F 2/4425 20130101; A61F 2002/30909
20130101; A61F 2310/00023 20130101; A61F 2220/0041 20130101; A61F
2002/30433 20130101; A61F 2002/30594 20130101; A61F 2002/30518
20130101; A61F 2002/3092 20130101; A61F 2310/00365 20130101; A61F
2002/30507 20130101; A61F 2230/0065 20130101; A61F 2002/302
20130101; A61F 2002/30769 20130101; A61F 2/442 20130101; A61F
2002/30975 20130101; A61F 2002/305 20130101; A61F 2/30742 20130101;
A61F 2002/30451 20130101; A61F 2002/30331 20130101; A61F 2002/30171
20130101; A61F 2220/0025 20130101; A61F 2002/443 20130101; A61F
2310/00017 20130101; A61F 2/30767 20130101; A61F 2250/0006
20130101; A61F 2230/005 20130101; A61F 2002/30492 20130101 |
Class at
Publication: |
623/17.13 |
International
Class: |
A61F 002/44 |
Claims
We claim:
1. An intervertebral spacer device, comprising: first and second
plates, each having an outwardly facing plate surface and an
inwardly facing plate surface, the plates being disposed such that
the inwardly facing plate surfaces are directed toward one another
and the outwardly facing plate surfaces are directed away from one
another; and at least one restoring force providing element
disposed between said inwardly facing surfaces, and disposed such
that a load applied to the outwardly facing surfaces is
counteracted by the at least one restoring force providing element;
the at least one restoring force providing element including a
belleville washer having a first end and a second end, the
belleville washer being rigidly fixed at the first end to the
inwardly facing surface of the first plate and rotationally mounted
at the second end to the inwardly facing surface of the second
plate.
2. The intervertebral spacer device of claim 1, further comprising
a post structure extending from the inwardly facing surface of the
second plate, the post structure having a central longitudinal axis
and a head, and wherein the belleville washer has a central opening
at the second end of the belleville washer, through which the head
is mountable to allow the second end of the belleville washer to
rotate about the central longitudinal axis and to limit other
movement of the second end of the belleville washer relative to the
central longitudinal axis.
3. The intervertebral spacer device of claim 2, wherein the post
structure is deflectable toward the central longitudinal axis into
a deflected state upon the application of a corresponding force,
and returnable to an undeflected state upon relief from the force,
and wherein the central opening of the belleville washer has a
diameter, the post structure has a trunk having a diameter smaller
than the diameter of the central opening, the head of the post
structure has a diameter greater than the diameter of the central
opening in the undeflected state of the post structure and a
diameter smaller than the diameter of the central opening in the
deflected state of the post structure, the belleville washer has a
thickness adjacent the central opening, and the trunk has a length
greater than the thickness of the belleville washer adjacent the
central opening.
4. The intervertebral spacer device of claim 3, wherein the post
structure comprises a plurality of post members separated from one
another by a plurality of slots, each of the post members being
deflectable toward one another, each of the post members having a
member trunk and a member head, the member trunks collectively
forming the trunk of the post structure, the member heads
collectively forming the head of the post structure.
5. The intervertebral spacer device of claim 3, wherein deflection
of the post structure under the force is preventable.
6. The intervertebral spacer device of claim 5, wherein the post
structure comprises a central threaded bore adapted to receive a
screw, and disposition of the screw within the central threaded
bore prevents the post from deflecting toward the central
longitudinal axis of the post structure, and withdrawal of the
screw from the central threaded bore allows the post structure to
deflect toward the central longitudinal axis of the post
structure.
7. The intervertebral spacer device of claim 1, wherein the
belleville washer has a spiral slot.
8. The intervertebral spacer device of claim 7, wherein the spiral
slot extends from a locus adjacent a peripheral edge of the
belleville washer to a locus which is radially in from the
peripheral edge.
9. The intervertebral spacer device of claim 8, wherein the spiral
slot extends for at least a 360 degree turn.
10. The intervertebral spacer device of claim 9, wherein the spiral
slot extends for a more than 360 degree turn.
11. The intervertebral spacer device of claim 3, wherein the second
end has a flat portion in which the central opening is formed, the
flat portion having a diameter greater than the diameter of the
head that is greater than the diameter of the central opening in
the undeflected state of the post structure.
12. The intervertebral spacer device of claim 1, wherein each of
the outwardly facing surfaces has a porous feature suitable for
bone ingrowth.
13. The intervertebral spacer device of claim 12, wherein the
porous feature comprises a wire mesh.
14. An intervertebral spacer device, comprising: first and second
plates, each having an outwardly facing surface and a inwardly
facing surface, the plates being disposed such that the inwardly
facing surfaces face one another; a spring having a first end and a
second end and a longitudinal axis, the first end being rigidly
mounted to the first plate, the second end being mounted to the
second plate for rotational movement of the second end of the
spring about the longitudinal axis relative to the second plate and
for limited other movement of the second end of the spring relative
to the second plate.
15. The intervertebral spacer device of claim 14, wherein the
spring comprises a belleville washer.
16. The intervertebral spacer device of claim 15, wherein the first
end of the belleville washer is a wide end of the belleville washer
and the second end of the belleville washer is a narrow end of the
belleville washer.
17. The intervertebral spacer device of claim 16, wherein the
belleville washer has at least one spiral slot.
18. The intervertebral spacer device of claim 17, wherein the
spiral slot extends from a locus adjacent a peripheral edge of the
belleville washer to a locus which is radially in from the
peripheral edge.
19. The intervertebral spacer device of claim 14, wherein the
second plate comprises a central structure extending from the
inwardly facing surface of the second plate, the central structure
having a radially outwardly extending ledge parallel to the
inwardly facing surface of the second plate, and wherein the second
end of the spring has a central opening through which the central
structure is disposable to seat the second end of the spring
between the ledge and the inwardly facing surface of the second
plate, such that interference of the second end of the spring with
the ledge and the inwardly facing surface of the second plate
limits movement of the second end of the spring along the
longitudinal axis relative to the second plate.
20. The intervertebral spacer device of claim 19, wherein a minimum
distance between the ledge and the inwardly facing surface of the
second plate accommodates a maximum thickness of the second end of
the spring such that the spring is free to rotate about the
longitudinal axis relative to the second plate.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 09/789,936, filed Feb. 15, 2001, which is
fully incorporated herein by reference.
FIELD OF THE INVENTION
[0002] This invention relates generally to a spinal implant
assembly for implantation into the intervertebral space between
adjacent vertebral bones to simultaneously provide stabilization
and continued flexibility and proper anatomical motion, and more
specifically to such a device which utilizes a spirally slotted and
rotatably mounted belleville washer as a restoring force generating
element.
BACKGROUND OF THE INVENTION
[0003] The bones and connective tissue of an adult human spinal
column consists of more than 20 discrete bones coupled sequentially
to one another by a tri-joint complex which consists of an anterior
disc and the two posterior facet joints, the anterior discs of
adjacent bones being cushioned by cartilage spacers referred to as
intervertebral discs. These more than 20 bones are anatomically
categorized as being members of one of four classifications:
cervical, thoracic, lumbar, or sacral. The cervical portion of the
spine, which comprises the top of the spine, up to the base of the
skull, includes the first 7 vertebrae. The intermediate 12 bones
are the thoracic vertebrae, and connect to the lower spine
comprising the 5 lumbar vertebrae. The base of the spine is the
sacral bones (including the coccyx). The component bones of the
cervical spine are generally smaller than those of the thoracic
spine, which are in turn smaller than those of the lumbar region.
The sacral region connects laterally to the pelvis. While the
sacral region is an integral part of the spine, for the purposes of
fusion surgeries and for this disclosure, the word spine shall
refer only to the cervical, thoracic, and lumbar regions.
[0004] The spinal column of bones is highly complex in that it
includes over twenty bones coupled to one another, housing and
protecting critical elements of the nervous system having
innumerable peripheral nerves and circulatory bodies in close
proximity. In spite of these complications, the spine is a highly
flexible structure, capable of a high degree of curvature and twist
in nearly every direction.
[0005] Genetic or developmental irregularities, trauma, chronic
stress, tumors, and degenerative wear are a few of the causes that
can result in spinal pathologies for which surgical intervention
may be necessary. A variety of systems have been disclosed in the
art which achieve immobilization and/or fusion of adjacent bones by
implanting artificial assemblies in or on the spinal column. The
region of the back which needs to be immobilized, as well as the
individual variations in anatomy, determine the appropriate
surgical protocol and implantation assembly. With respect to the
failure of the intervertebral disc, the interbody fusion cage has
generated substantial interest because it can be implanted
laparoscopically into the anterior of the spine, thus reducing
operating room time, patient recovery time, and scarification.
[0006] Referring now to FIGS. 1 and 2, in which a side perspective
view of an intervertebral body cage and an anterior perspective
view of a post implantation spinal column are shown, respectively,
a more complete description of these devices of the prior art is
herein provided. These cages 10 generally comprise tubular metal
body 12 having an external surface threading 14. They are inserted
transverse to the axis of the spine 16, into preformed cylindrical
holes at the junction of adjacent vertebral bodies (in FIG. 2 the
pair of cages 10 are inserted between the fifth lumbar vertebra
(L5) and the top of the sacrum (S1). Two cages 10 are generally
inserted side by side with the external threading 14 tapping into
the lower surface of the vertebral bone above (L5), and the upper
surface of the vertebral bone (S1) below. The cages 10 include
holes 18 through which the adjacent bones are to grow. Additional
material, for example autogenous bone graft materials, may be
inserted into the hollow interior 20 of the cage 10 to incite or
accelerate the growth of the bone into the cage. End caps (not
shown) are often utilized to hold the bone graft material within
the cage 10.
[0007] These cages of the prior art have enjoyed medical success in
promoting fusion and grossly approximating proper disc height. It
is, however, important to note that the fusion of the adjacent
bones is an incomplete solution to the underlying pathology as it
does not cure the ailment, but rather simply masks the pathology
under a stabilizing bridge of bone. This bone fusion limits the
overall flexibility of the spinal column and artificially
constrains the normal motion of the patient. This constraint can
cause collateral injury to the patient's spine as additional
stresses of motion, normally borne by the now-fused joint, are
transferred onto the nearby facet joints and intervertebral discs.
It would therefore, be a considerable advance in the art to provide
an implant assembly which does not promote fusion, but, rather,
which nearly completely mimics the biomechanical action of the
natural disc cartilage, thereby perimitting continued normal motion
and stress distribution.
[0008] It is, therefore, an object of the present invention to
provide a new and novel intervertebral spacer that stabilizes the
spine without promoting a bone fusion across the intervertebral
space.
[0009] It is further an object of the present invention to provide
an implant device which stabilizes the spine while still permitting
normal motion.
[0010] It is further can object of the present invention to provide
a device for implantation into the intervertebral space that does
not promote the abnormal distribution of biomechanical stresses on
the patient's spine.
[0011] Other objects of the present invention not explicitly stated
will be set forth and will be more clearly understood in
conjunction with the descriptions of the preferred embodiments
disclosed hereafter.
SUMMARY OF THE INVENTION
[0012] The preceding objects of the invention are achieved by the
present invention which is a flexible intervertebral spacer device
comprising a pair of spaced apart base plates, arranged in a
substantially parallel planar alignment (or slightly offset
relative to one another in accordance with proper lordotic
angulation) and coupled to one another by means of a spring
mechanism. In particular, this spring mechanism provides a strong
restoring force when compression and/or lateral deflection loads
are applied to the plates, and also permits rotation of the two
plates relative to one another. While there are a wide variety of
embodiments contemplated, a preferred embodiment includes a
belleville washer utilized as the restoring force providing
element, the belleville washer being spirally slotted and rotatably
mounted.
[0013] More particularly, as the assembly is to be positioned
between the facing surfaces of adjacent vertebral bodies, the base
plates should have substantially flat external surfaces that seat
against the opposing bone surfaces. In as much as these bone
surfaces are often concave, it is anticipated that the opposing
plates may be convex in accordance with the average topology of the
spinal anatomy. In addition, the plates are to mate with the bone
surfaces in such a way as to not rotate relative thereto. (The
plates rotate relative to one another, but not with respect to the
bone surfaces to which they are each in contact with.) In order to
prevent rotation of a plate relative to its adjacent bone, the
upper and lower plates alternatively may each include outwardly
directed spikes or ridges that penetrate the bone surface and
mechanically hold the plates in place. However, it is more
preferably anticipated that the plates should include a porous
feature into which the bone of the vertebral body can grow. The
most desirable upper and lower plate surface porous feature is a
deflectable wire mesh into which the bone can readily grow, and
which mesh will deform to seat into the concave upper and lower
bone faces. (Note that this limited fusion of the bone to the base
plate does not extend across the intervertebral space.) These
features, while being preferred, are not required.
[0014] In some embodiments (although not in the preferred
embodiment), between the base plates, on the exterior of the
device, there is included a circumferential wall which is resilient
and which simply prevents vessels and tissues from entering within
the interior of the device This resilient wall may comprise a
porous fabric or a semi-impermeable elastomeric material. Suitable
tissue compatible materials meeting the simple mechanical
requirements of flexibility and durability are prevalent in a
number of medical fields including cardiovascular medicine, wherein
such materials are utilized for venous and arterial wall repair, or
for use with artificial valve replacements. Alternatively, suitable
plastic materials are utilized in the surgical repair of gross
damage to muscles and organs. Still further materials that could be
utilized herein may be found in the orthopedic field in conjunction
with ligament and tendon repair. It is anticipated that future
developments in this area will produce materials that are
compatible for use with this invention, the breadth of which shall
not be limited by the choice of such a material.
[0015] As introduced above, the internal structure of the present
invention comprises a spring member, which provides a restoring
force when compressed or laterally deflected. The restoring force
providing subassembly does not substantially interfere with the
rotation of the opposing plates relative to one another. In the
preferred embodiment, the spring subassembly is configured to allow
rotation of the plates relative to one another. As further
mentioned above, the force restoring member comprises at least one
belleville washer that is spirally slotted.
[0016] Belleville washers are washers that are generally bowed in
the radial direction. Specifically, they have a radial convexity
(i.e., the height of the washer is not linearly related to the
radial distance, but may, for example, be parabolic in shape). The
restoring force of a belleville washer is proportional to the
elastic properties of the material. In addition, the magnitude of
the load support and restoring force provided by the belleville
washer under compression and/or lateral deflection may be modified
by providing one or more slots in the washer. In the preferred
embodiment of the present invention, the belleville washer utilized
as the load supporting and force restoring member is spirally
slotted, with a single spiral slot initiating near the periphery of
the washer and extending along an arc that is radially inwardly
directed a distance toward the center of the bowed disc, and
terminating near the center of the bowed disc. Preferably, the
spiral slot extends around the surface of the belleville washer for
more than 360 degrees and preferably 540 degrees. Additional
configurations, including multiple slots, arcs of different lengths
and/or arcs that extend for more or less than 360 degrees, can be
used to adjust the load bearing and force restoring characteristics
of the belleville washer within the scope of the present
invention.
[0017] In the preferred embodiment of the present invention, a
single belleville washer, which is slotted as described above, is
utilized in conjunction with a rotational mounting between one end
of the belleville washer and one of the opposing plates, and a
rigid fixation of the other end of the belleville washer to the
other of the opposing plates. The rotational mounting allows the
washer to freely rotate relative to the one of the opposing plates.
In as much as the washer is rigidly fixed to the other of the
opposing plates, the mounting allows the opposing plates to rotate
relative to one another. More particularly, this embodiment
comprises a pair of spaced apart base plates, one of which is a
disc shaped member (preferably shaped to match the end of an
intervertebral disc) having an external face (preferably having the
porous coating discussed above) and an internal face. The wide end
of the belleville washer is rigidly fixed to the internal face of
this base plate, preferably by laser welding. The other of the base
plates is similarly shaped, having an exterior face (preferably
having the porous coating discussed above), but further includes on
its internal face a central post which rises out of the internal
face at a nearly perpendicular angle (it should be understood that
the post need not extend from the center of the plate, but rather
is can be positioned according to the proper clinical placement
depending on where the device is placed in the spine, In as much as
a more anterior or a more posterior position may be suitable in
certain parts of the spine). The central post comprises a plurality
of upwardly extending members that mutually define a cylinder
having a central axial bore and vertically oriented slots
separating each individual member. This conformation permits the
mutually defined cylindrical shape to deflect inward upon the
application of a corresponding force and return to an undeflected
shape once the force is relieved. Each of the upwardly extending
members comprises a generally uniform radial thickness, thereby
mutually defining a constant diameter for the cylinder from its
union with the plate up to a circumferential position near to the
uppermost extent thereof. The uppermost extent thereof, however,
comprises a discontinuously widened circumference that subsequently
tapers radially inwardly from that vertical position to the upper
end. This discontinuously widened circumference thereby defines an
annular ledge around the cylindrical top section, which ledge
tapers inwardly to provide a beveled conformation. The portion of
the post from the ledge to the upper end of the post is referred to
herein as the head of the post. The central axial bore is threaded,
and receives a small set screw. Prior to the insertion of the set
screw, the post can deflect radially inward because of the axial
bore and the vertically oriented slots. The insertion of the set
screw eliminates the capacity for this deflection
[0018] As introduced above, the spirally slotted belleville washer
is mounted to this central post in such a way that it may rotate
freely through a range of rotation angles equivalent to the
fraction of normal human spine rotation (to mimic normal disc
rotation). In this regard, the belleville washer has a flattened
narrow end with a central opening. The central opening has a
diameter that is greater than the diameter of the post up to the
ledge, but smaller than the diameter of the head at the ledge.
Therefore, the head can be passed through the central opening when
the set screw is not in the axial bore, because the slots will
allow the head to deflect inward when the head is forced through
the central opening. Once the head has passed through the central
opening, the head will return to its undeflected shape so that the
narrow end is seated between the plate and the ledge. Subsequent
introduction of the set screw into the axial bore eliminates the
capacity for the head to deflect. Preferably, the length of the
post from the plate to the ledge is slightly longer than the
thickness of the washer at the narrow end, so that the washer is
restricted from angulating with respect to the plate but not
restricted from rotating with respect to the plate. (Angulation of
the plates relative to one another will be possible because of the
ability of the washer to deflect under lateral deflection forces
and return to its undeflected shape.)
[0019] The assembly provides ample spring-like performance with
respect to compression and lateral deflection loads, as well as
long cycle life to mimic the biomechanical performance of the
normal human intervertebral disc. The rigid fixation of the wide
end of the belleville washer maintains the wide end against the one
opposing plate. While the narrow end of the belleville washer can
rotate freely relative to the other opposing plate, the narrow end
is angulationally fixed relative to that plate (as described
above). Therefore, not only compression, but also lateral
deflection loads, are borne by the washer spring. The spiral slot
of the belleville washer allows the washer to compress as the slot
narrows under compression loads, only to spring back into its
undeflected shape upon the unloading of the spring. Further, the
spiral slot allows one side of the washer to compress and the
opposite side to expand as the portion of the slot on the one side
narrows and the portion of the slot on the opposite side widens
under lateral deflection loads, only to spring back into its
undeflected shape upon the unloading of the spring.
[0020] Finally, In as much as the human body has a tendency to
produce fibrous tissues in perceived voids, such as may be found
within the interior of the present invention, and such fibrous
tissues may interfere with the stable and/or predicted functioning
of the device, some embodiments of the present invention (although
not the preferred embodiment) will be filled with a highly
resilient elastomeric material. The material itself should be
highly biologically inert, and should not substantially interfere
with the restoring forces provided by the spring-like mechanisms
therein. Suitable materials may include hydrophilic monomers such
as are used in contact lenses. Alternative materials include
silicone jellies and collagens such as have been used in cosmetic
applications. As with the exterior circumferential wall, which was
described above as having a variety of suitable alternative
materials, it is anticipated that future research will produce
alternatives to the materials described herein, and that the future
existence of such materials which may be used in conjunction with
the present invention shall not limit the breadth thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a side perspective view of an interbody fusion
device of the prior art.
[0022] FIG. 2 is a front view of the anterior portion of the
lumbo-sacral region of a human spine, into which a pair of
interbody fusion devices of the type shown in FIG. 1 have been
implanted.
[0023] FIGS. 3a and 3b are side cross-section views of the upper
and lower opposing plates of the preferred embodiment of the
present invention.
[0024] FIGS. 4a and 4b are top and side cross-section view of a
belleville washer having a spiral slot, for use in a preferred
embodiment of the present invention.
[0025] FIG. 5a is a top view of the upper plate of FIG. 3a, with
the wide end of the belleville washer of FIGS. 4a and 4b rigidly
fixed to the upper plate.
[0026] FIG. 5b is a top view of the lower plate of FIG. 3b.
[0027] FIG. 6 is a side cross-section view of the preferred
embodiment of the present invention, which utilizes a belleville
washer of the type shown in FIGS. 4a and 4b, showing the plates of
FIGS. 6a and 6b assembled together.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0028] While the present invention will be described more fully
hereinafter with reference to the accompanying drawings, in which
particular embodiments and methods of implantation are shown, it is
to be understood at the outset that persons skilled in the art may
modify the invention herein described while achieving the functions
and results of this invention. Accordingly, the descriptions that
follow are to be understood as illustrative and exemplary of
specific structures, aspects and features within the broad scope of
the present invention and not as limiting of such broad scope. Like
numbers refer to similar features of like elements throughout.
[0029] Referring now to FIGS. 3a and 3b, side cross-section views
of upper and lower plate members 100, 200 of the preferred
embodiment of the present invention are shown. As the device is
designed to be positioned between the facing surfaces of adjacent
vertebral bodies, the plates include substantially flat external
face portions 102, 202 that seat against the opposing bone
surfaces. In addition, the plates are to mate with the bone
surfaces in such a way as to not rotate relative thereto. (The
plates rotate relative to one another, but not with respect to the
bone surfaces with which they are each in contact.) In order to
prevent rotation of each plate relative to its adjacent bone, it
is, therefore, preferred that the external faces of the plates
include a porous feature 104, 204 into which the bone of the
vertebral body can grow. The most desirable upper and lower plate
surface porous feature is a deflectable wire mesh 104,204 into
which the bone can readily grow, and which mesh will deform to seat
into the concave upper and lower bone faces. (Note that this
limited fusion of the bone to the base plate does not extend across
the intervertebral space.) A hole (not shown) can be provided in
the upper plate such that the interior of the device may be readily
accessed if a need should arise.
[0030] The upper plate 100 includes an internal face 103. The lower
plate 200 includes an internal face 203 that includes a central
post 201 which uses out of the internal face 203 at a nearly
perpendicular angle. The central post 201 comprises a plurality of
upwardly extending members 202 which mutually define a cylinder 201
having a central axial bore 209 and vertically oriented slots 206
separating each individual member 202. This conformation permits
the cylinder 201 to deflect inward upon the application of a
corresponding force and return to an undeflected shape once the
force is relieved. Each of the upwardly extending members 202
comprises a generally uniform radial thickness, thereby mutually
defining a constant diameter for the cylinder 201 from its union
204 with the internal face 203 of the lower plate 200 up to a
circumferential position 208 near to the uppermost extent 210
thereof. The uppermost extent 210 thereof, however, comprises a
discontinuously widened circumference which subsequently tapers
radially inwardly from that vertical position to the upper end of
the members 202. This discontinuously widened circumference thereby
defines an annular ledge 212 around the cylindrical top section,
which ledge 212 tapers inwardly to provide a beveled conformation.
The portion of the post 201 from the ledge 212 to the upper end of
the post 201 is referred to herein as the head 207 of the post 201.
The central axial bore 209 is threaded and is designed to receive a
set screw 205. Prior to the insertion of the set screw 205, the
post 201 can deflect radially inward because of the axial bore 209
and the vertically oriented slots 206. The insertion of the set
screw 205 eliminates the capacity for this deflection.
[0031] Referring now to FIGS. 4a and 4b, a spirally slotted
belleville washer 130 is provided in top and side cross-section
views, respectively. The belleville washer 130 is a restoring force
providing device which comprises a circular shape, having a wide
end 139 and a flattened narrow end 133 with a central opening 132,
and which is radially arched in shape. The belleville washer 130
has a radial convexity (i.e., the height of the washer 130 is not
linearly related to the radial distance, but may, for example, be
parabolic in shape). The restoring force of the belleville washer
130 is proportional to the elastic properties of the material.
[0032] The belleville washer 130 has a spiral slot 131 formed
therein. The slot 131 extends from a point near the periphery of
the wide end 139 of the washer 130, along an arc that is radially
inwardly directed a distance toward a the center of the washer 130,
and terminates at a point near the central opening 132, preferably
where the flatness of the flattened narrow end 133 begins. In
preferred embodiments, the slot 131 extends around the surface of
the belleville washer 130 for more than 360 degrees, and most
preferably, for 540 degrees as shown. Additional configurations,
including multiple slots, arcs of different lengths and/ or arcs
that extend for more or less than 360 degrees, can be used to
adjust the load bearing and force restoring characteristics of the
belleville washer 130 within the scope of the present invention,
depending upon the requirements of the patient, and the anatomical
requirements of the device.
[0033] The central opening 132 of the belleville washer 130 is
dimensioned to receive the head 207 of the post 201 of the lower
plate 200 described above. More particularly, the diameter of the
central opening 132 is greater than the diameter of the cylinder
201 from the union 204 with the internal face 203 of the lower
plate 200 up to the ledge 212, but smaller than the diameter of the
head 207 at the ledge 212. Therefore, the head 207 can be passed
through the central opening 132 when the set screw 205 is not in
the axial bore 209, because the slots 206 will allow the head 207
to deflect inward when the head 207 is forced through the central
opening 132. Once the head 207 has passed through the central
opening 132, and consequently the force causing the deflection of
the head 207 is relieved, the head 207 will return to its
undeflected shape so that the narrow end 133 is seated between the
internal face 203 of the lower plate 200 and the ledge 212 of the
post 201. Subsequent introduction of the set screw 205 into the
axial bore 209 eliminates the capacity for the head 207 to deflect,
ensuring that the head 207 cannot back through the opening 132
without removal of the set screw 205. Preferably, as shown, the
length of the post 201 from the internal face 203 of the lower
plate 200 to the ledge 212 is slightly larger than the thickness of
the washer 130 at the narrow end 133, so that the washer 130 is
restricted from angulating with respect to the lower plate 200 but
not restricted from rotating with respect to the lower plate 200.
(Angulation of the plates relative to one another will be possible
because of the ability of the washer 130 to deflect under lateral
deflection forces and return to its undeflected shape.) It should
be noted that the flat configuration of the narrow end 133 of the
washer 130 facilitates this preferable fitting of the narrow end
133 between the ledge 212 and the plate 200.
[0034] Referring now to FIG. 5a, a top view of the upper plate 100
of FIG. 3a, with the wide end 139 of the spirally slotted
belleville washer 130 of FIGS. 4a and 4b rigidly secured thereto,
preferably by laser welding the wide end 139 to the upper plate
100, is shown. FIG. 5b shows a top view of the lower plate 200 of
FIG. 3b, showing the set screw 205 in the axial bore 209 of the
post 201.
[0035] FIG. 6 shows the fully assembled preferred embodiment of the
present invention. The spirally slotted belleville washer 130 of
FIGS. 4a and 4b is placed with its wide end rigidly fixed against
the top plate 100 as shown in FIG. 5a. The head 207 of the post 201
of the lower plate 200 is fitted into the central opening 132 of
the belleville washer 130 as described above, so that the washer
130 cannot be readily removed therefrom, but can still rotate
thereon. Thereafter, the device can be placed between two vertebral
bodies, with the porous features 104, 204 facilitating bore growth
thereinto and securing the plates 100, 200 to the adjacent bones.
Loading of the assembly under normal motion causes the washer 130
to deflect (with the spiral slot 131 enhancing the deflection).
More particularly, the spiral slot of the belleville washer allows
the washer to compress as the slot narrows under compression loads,
only to spring back into its undeflected shape upon the unloading
of the spring. Further, the spiral slot allows one side of the
washer to compress and the opposite side to expand as the portion
of the slot on the one side narrows and the portion of the slot on
the opposite side widens under lateral deflection loads, only to
spring back into its undeflected shape upon the unloading of the
spring.
[0036] In as much as the human body has a tendency to produce
fibrous tissues in perceived voids, such as may be found within the
interior of the present invention, and such fibrous tissues may
interfere with the stable and/or predicted functioning of the
device, some embodiments of the present invention (although not the
preferred embodiment) will be filled with a highly resilient and
biologically inert elastomeric material. Suitable materials may
include hydrophilic monomers such as are used in contact lenses.
Alternative materials include silicone jellies and collagens such
as have been used in cosmetic applications.
[0037] While there has been described and illustrated specific
embodiments of an intervertebral spacer device, it will be apparent
to those skilled in the art that variations and modifications are
possible without deviating from the broad spirit and principle of
the present invention. The invention, therefore, shall not be
limited to the specific embodiments discussed herein.
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